1. Field of the Invention
The present invention relates to a fiber-reinforced light metal alloy piston for internal combustion engines.
2. Description of the Related Art
It is well known to manufacture internal combustion engine pistons from light metal alloy castings such as aluminum alloys. Since light metal alloys have a larger coefficient of linear expansion as compared with steel alloys, the skirt section of the light metal alloy piston is subjected to considerable thermal deformation between the cold start condition and the warmed up condition of the engine. If the piston skirt section is so sized as to provide little clearance between the outer periphery thereof and the inner surface of the cylinder bore during cold start of the engine, then the friction between the piston skirt and the cylinder bore would become prohibitively high when the engine is warmed up, since the piston clearance in the bore is reduced due to thermal expansion of the piston skirt section. Conversely, if the clearance is large enough to avoid the above-mentioned problem, then the engine will generate piston slap to an unacceptable level during cold start of the engine, because of the excessive clearance between the piston shirt and cylinder bore. In order to meet these opposing requirements, it is desirable to suppress thermal expansion of the light metal alloy piston skirt section so that an optimum clearance is maintained regardless of the engine temperature.
One solution known in the art is to thermally isolate the skirt section from the heated piston crown section by means of a plurality of slits extending through the wall of the skirt perpendicular to the longitudinal axis of the piston. These slits communicate the oil ring groove with the inside of the piston and are primarily intended as oil passages serving to direct oil scraped from the surface of the cylinder bore by the oil control ring toward the interior of the piston. These slits have been found to act as a heat dam that prevents the transfer of heat from the piston crown to the skirt section. However, in supercharged high-speed high-power engines, the pistons tend to be subjected to increasingly high heat loads. Therefore, in such high power engines, it is desirable to dissipate heat through the piston skirt section, although most of the heat received by the piston crown from the combustion chamber is primarily transferred through piston rings to the engine cylinders. For this reason, the recent trend in high power engines is to reduce or even abolish the heat dam slits located between the piston crown and the skirt section. This causes the temperature of the skirt section to be elevated by 30.degree. to 40.degree. C. as compared with conventional non-supercharged engines, resulting in considerable thermal deformation of the skirt section.
Another solution is to provide within the skirt section a steel ring known as a "thermal strut" and having a high tensile strength sufficient to prevent thermal expansion of the piston skirt. The thermal strut is in the form of an insert and is molded within the matrix of the light metal alloy by an insert casting technique. The disadvantage of such a steel thermal strut is that it increases the weight of the piston and, thus, becomes a bar to designing light weight pistons.
It has been proposed, therefore, to use thermal struts made from fiber reinforced light metal alloys, instead of steel thermal struts, as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 59-229033 and 59-229034, and Japanese Unexamined Utility Model Publication (Kokai) Nos. 60-12650, 60-28246, 60-28247, and 60-28248. The thermal strut of fiber reinforced light metal alloys comprises a circumferentially wound bundle of high-tensile-strength inorganic fibers, such as carbon fibers and silicon carbide fibers, which are integrally molded within a matrix light metal alloy to form an annular fiber-reinforced portion within the confinement of the shoulder portion of the skirt section. In the fiber reinforced portion, individual fibers are firmly bonded to the matrix metal. Due to the low coefficient of linear thermal expansion of the high tensile strength fibers, the annular fiber-reinforced portion serves as a thermal strut which precludes thermal expansion of the shoulder portion of the skirt section.
However, the problem which must be overcome in the manufacture of light-metal-alloy casted pistons having thermal struts comprising inorganic reinforcing fibers is that cracks are formed in the matrix metal of the skirt shoulder portion in the vicinity of the boundary of the fiber reinforced metal portion due to the difference between the linear expansion coefficient of the fibers and that of the matrix light metal alloy. For example, the coefficient of linear expansion of aluminum alloy is in the order of 20.times.10.sup.-6 /.degree.C., and that of carbon fibers is about -1.2.times.10.sup.-6 /.degree.C. This means that, when the piston is repeatedly heated and cooled in response to engine stopping and restarting, the matrix metal located in the non-fiber-reinforced portion adjacent to the fiber-reinforced portion undergoes a considerable amount of repeated expansion and contraction, whereas the matrix metal located within the fiber reinforced portion remains substantially free from such expansion because of restraint by the reinforcing fibers. As a result, the matrix metal in the non-reinforced portion is subjected to a large stress which gives rise to cracks along the boundary of the fiber reinforced portion, as described later in more detail with reference to the drawings.
Another problems involved in light metal alloy pistons having fiber reinforced thermal struts arises from the recent requirement that the axial length of the piston be reduced. To meet this requirement, piston pin receiving bores machined in piston pin bosses must be located as close to the skirt shoulder portion as possible. This necessarily results in the thermal strut being cut out by machining of piston pin receiving bores, whereby the reinforcing fibers are exposed to the pin receiving bores. This presents the following disadvantages. First, since the fiber reinforced thermal strut is cut out at an acute angle, the ends of the strut are exposed within the piston pin receiving bores in a cantilever fashion to form sharp edges. As is well known, although reinforcing fibers such as carbon fibers exhibit a high tensile strength against an effort applied in the lengthwise direction thereof, they nevertheless have poor resistance against bending stress that is applied in the transverse direction. As the skirt shoulder is repeatedly compressed in the axial direction due to power pulses during operation of the engine, carbon fibers at the exposed edges of the thermal strut are broken and are removed from the matrix metal alloy. This causes cracks to occur, originating from the broken edges, and reduces the service life of the piston.
A second disadvantage resides in the difficulties in machining the piston pin receiving bore. Since the pin receiving bores are intended to slidingly engage with the piston pin, the inner surface of the bores must be machined to present a certain surface roughness. To this end, after the bores are drilled through the pin bosses, the bore surface is subjected to grinding. However, it has been difficult to obtain the desired surface roughness when the pin receiving bores intersect with the fiber reinforced thermal strut because machining of the carbon fiber is not feasible.